Conventionally, a technology relating to a feedback control for output voltage in power circuit has been known and one of them is disclosed in Japanese Patent Application Laid-open No. 6-284590.
It has been proposed by installing a high voltage battery (36V) and a low voltage battery (12V) in a vehicle to drive a motor for vehicle driving as well as to drive low voltage electric equipment system. A system that charges the low voltage battery by dropping the voltage of the high voltage battery by using a DC—DC converter when the voltage of low voltage battery drops has been adopted to the vehicle. In this system, the low voltage battery can be charged under a stable condition by feedback controlling the output voltage of the DC—DC converter.
However, when the voltage of the high voltage battery is suddenly varied under an engine starting, the feedback control of the output voltage cannot cope with such sudden voltage change of the high voltage battery. Therefore, the varied voltage may be inputted to the low voltage battery, although intended to input a predetermined voltage.